This invention relates to presses of the type wherein two pressure platens are each supported by a support construction so that the forces of each of the press platens are transmitted to their corresponding support construction and in which means are provided to regulate spacing of the press platens including pressure elements positioned between the platen and respective support construction and means provided to regulate the pressure therein, in general, and more particularly, to a safety device for such a press which prevents excessive pressures developing therein.
A press of this general nature is disclosed in U.S. Pat. No. 3,851,658, the disclosure of which is hereby incorporated by reference. Means for maintaining constant spacing in such a press are described in detail in U.S. Pat. No. 3,881,852 which is also hereby incorporated by reference. The press disclosed in these references is a continuous press for the manufacture of wood, chipped wood or the like in which the material to be treated is moved continuously between two flexible endless conveyor belt spans. Between the belt span forming the press zone and the press platens, a plurality of rotatively unpowered endless loops of roller chains are used, these roller chains being packed transversely together to form a bed interposed between the platens and the seal strip conveyor belts. The press platens in turn are positioned between support constructions comprising beam, each positioned transversely to the movement of the belts with portions of such beams extending longitudinally above and below the press platens for the length of a press. Hydraulic pressure cushions or hydraulic cylinders are interposed between the beams and pressure platen to control pressure and maintain the proper spacing. In the second of the above-mentioned references, means for controlling this spacing in the form of a guiding valve arranged so that when the distance between the press platens is at a pre-determined spacing, the valve is inactive and is closed. A pressure pump coupled through the valve causes an increase in the pressure in the pressure elements until the press platens begin to move together changing their position. At that point, the guide valve opens to allow a bleed-off of hydraulic fluid relieving the pressure so that the press platen moves back in a direction to cause it again to be at the pre-determined distance, where upon reaching the pre-determined distance, the valve becomes inactive. Through this arrangement, a dynamic balance about the pre-determined distance between the platens is achieved. Such a design works quite well. However, if the material being compressed for some reason presents to the press a greater resistance to compression than is normal, the pressure in the pressure elements increases. In other words, it increases to maintain the pre-determined spacing. In the press described in the aforementioned Patents where strips of wood chip panels or the like are made, varying compressibility of the materials can result from non-uniformity of the chips supplied. In the zones of denser material, substantially higher pressure is needed to compress to the uniform specified thickness. The aforementioned valve insures that the proper spacing is maintained by increasing the pressure as required.
However, a situation can exist where the necessary pressure to achieve the desired thickness exceeds the stress limits of the machine and can result in structural damage to the machine or associated hydraulic lines. Thus, the need for a device to prevent such damage to the machine becomes evident.